Abstract:

Multiple sclerosis is a progressive autoimmune disease where myelin is gradually
stripped from axons. Axon degeneration inevitably follows protracted myelin loss
ultimately leading to irreversible neurological decline. To better understand the cellular
mechanisms associated with the axon loss phase of the disease, spinal cord axons
from the experimental autoimmune encephalomyelitis (EAE) animal model of multiple
sclerosis were examined using correlated in vivo time-lapse microscopy and serial
section transmission electron microscopic (ssTEM) reconstruction. A novel technique,
termed near infrared burning (NIRB), was developed that took advantage of a
femtosecond-pulsed mode locked laser’s ability to create photoconvertable fiducial
markers for routine identification of previously imaged axons for ssTEM reconstruction.
This combination of imaging techniques revealed the subcellular milieu that underlies
axon degeneration at both the light and electron microscopic level. In particular,
paranodal regions of axons in EAE animals contained a significantly higher population
of mitochondria with large rounded, electron lucid, vesiculated mitochondria with
unorganized cristae compared to controls. This effect was largely restricted to the
paranodal region and was not always associated with direct immune cell interaction or
myelin loss. Together, these results suggest a novel mechanism for axon degeneration
that is not only focal in nature, but decoupled with myelin loss in the EAE animal model
of multiple sclerosis.